GCN Circular 30914

V. D'Elia (SSDC & INAF-OAR), A. D'Ai (INAF-IASFPA), J.A. Kennea (PSU),
A. Tohuvavohu (U. Toronto), B. Sbarufatti (PSU), K.L. Page (U.
Leicester), A.P. Beardmore (U. Leicester), P.A. Evans (U. Leicester),
A. Melandri (INAF-OAB) and  report on behalf of the Swift-XRT team:
report on behalf of the Swift-XRT team:

We have analysed 5.3 ks of XRT data for the -detected burst GRB
210928A, from 67.4 ks to 262.9 ks after the   trigger. The data are
entirely in Photon Counting (PC) mode. Using 1636 s of PC mode data and
2 UVOT images, we find an enhanced XRT position (using the XRT-UVOT
alignment and matching UVOT field sources to the USNO-B1 catalogue):
RA, Dec = 245.51109, -17.94788 which is equivalent to:

RA (J2000): 16h 22m 02.66s
Dec(J2000): -17d 56' 52.4"

with an uncertainty of 4.4 arcsec (radius, 90% confidence). This
position is 6793.5 arcmin from the  position. 

The light curve can be modelled with a power-law decay with a decay
index of alpha=1.3 (+1.8, -1.3).

A spectrum formed from the PC mode data can be fitted with an absorbed
power-law with a photon spectral index of 1.4 (+0.6, -0.3). The
best-fitting absorption column is  2.342 (+3.358, -0.021) x 10^21
cm^-2, consistent with the Galactic value of 2.3 x 10^21 cm^-2
(Willingale et al. 2013). The counts to observed (unabsorbed) 0.3-10
keV flux conversion factor deduced from this spectrum  is 4.8 x 10^-11
(5.5 x 10^-11) erg cm^-2 count^-1. 

A summary of the PC-mode spectrum is thus:
Total column:	     2.342 (+3.358, -0.021) x 10^21 cm^-2
Galactic foreground: 2.3 x 10^21 cm^-2
Excess significance: <1.6 sigma
Photon index:	     1.4 (+0.6, -0.3)

The results of the XRT-team automatic analysis are available at
http://www.swift.ac.uk/xrt_products/00014850.

This circular is an official product of the Swift-XRT team.